Dear developers,
I am using root 6.05/02 and I was trying to explain the error propagation to student, but unfortunately root did not support me ! Here is what I did:

This means that root calculated the square root of the bin not the scale factor multiplied by the square root of the bin before scaling. The bug (feature) did not exists in the old root versions.

Then you will get the correct errors.
However, in my opinion, I think the scale by default should compute correctly the errors, independently if Sumw2 has been called or not. I’ll think about what is the best way to do this